Final answer:
On upper-level charts, the crossing of isotherms and isobars signifies horizontal temperature advection and can indicate weather system development. The troposphere is where most weather phenomena occur due to convection.
Step-by-step explanation:
On an upper-level chart where the isotherms cross the isobars (or contours), and temperature advection occurs, the atmosphere can undergo various phenomena, including the development of weather systems. When isotherms cross isobars, it usually suggests that there is horizontal temperature advection occurring—meaning that a mass of air with a different temperature is being moved horizontally, usually by the wind, from one area to another. This advection can lead to changes in weather patterns, such as the formation of fronts or the development of high or low-pressure systems.
The structure of our atmosphere consists mostly of the troposphere, stratosphere, mesosphere, and ionosphere. It's important to note that atmospheric circulation and weather phenomena like wind and storms are largely confined to the troposphere. This is directly related to the fact that air in the troposphere is heated by the Earth's surface and rises, leading to convection, which in turn, drives weather systems.